Cardiac pacemakers recognize autonomous cardiac activity by means of an electrode connected to the heart. These electric signals have a low frequency range (up to a maximum of 1 kHz), and have an amplitude varying from about 1 to 10 mV. In order to properly function cardiac pacemakers must not be impaired by a multitude of interfering signals especially when fulfilling their therapeutic function.
There are many currently existing protective circuits which reduce the influx of interfering signals on cardiac pacemakers. One such solution consists in the installation of a capacitor at the terminal connector pair which attenuates interfering signals of higher frequency.
Another solution for the cardiac pacemaker is an electronic circuit located after the input filter and the input amplifier whose switching action stimulates the heart by 100% in the presence of any continuous interfering signal. However, this latter method is achieved at a loss of autonomous cardiac activity detection. A more advanced protective circuit allows the detection of autonomous cardiac activity without switching over to 100% cardiac stimulation during continuous interference.
Cardiac pacemakers presently available which are equipped with the above described circuit variations, however, do not provide sufficient protection against the presence of pulsating or modulating interference signals. Such interfering signals can lead to the suppression of therapeutically indicated impulse stimulation produced by the cardiac pacemaker. In the existing circuit systems, these interfering signals are merely attenuated by an input capacitor and other additional internal filters.
See, e.g., The Institute for Radio Technology in Munich (Institut fur Rundfunktechnik, GmbH, "IRT") reports in EMV, 1988, 545-554 on "The susceptibility of cardiac pacemakers to electric interference caused by powerful radio transmission". This publication lists 34 various types of pacemakers subject to electronic interference. The results show, for example, levels of pacemaker disturbance at an interference voltage ranging from 0.05 V to 2 V at 30 kHz, and an interference voltage of 0.2 V to 17 V at 500 kHz.
The demand for pacemakers with interference elimination is greater than ever. This has been demonstrated by DIN/VDE- and CENELEC in their discussions and studies on standardization. The IRT in Munich requires, for example, a pacemaker disturbance voltage resistance capability of at least 16 V, also for modulated and pulsating interference signals in the frequency range from 30 kHz to 30 MHz. Accordingly, it is an objective of the present invention to provide a protective circuit which will simply meet present requirements of disturbance voltage strength.